The load placed on a work unit increases according to the travel speed or work conditions in some self-propelled work machines such as, for example, auger-type snow removers that are provided with an engine-driven work unit. An auger-type snow remover is a work machine in which snow is gathered and removed using an auger (work unit) at the front of the machine while the machine travels forward. As the travel speed increases, the amount of snow removed by the auger also increases. As a result, the load placed on the auger increases. This type of auger-type snow remover is described in Japanese Utility Model Laid-Open Publication No. 3-32617, and Japanese Patent Laid-Open Publication Nos. 2004-278055 and 2002-142307.
In the auger-type snow remover described in Japanese Utility Model Laid-Open Publication No. 3-32617, the travel speed of the travel unit is varied when the load placed on the auger changes according to the type or accumulated amount of snow.
In the auger-type snow remover described in Japanese Patent Laid-Open Publication No. 2004-278055, notification is given by an indicator lamp when the actual rotational speed of the engine increases or decreases in relation to a target rotational speed. The rotational speed of the engine is thereby varied according to fluctuation of the load exerted on the auger. An operator may therefore change the travel speed of the auger-type snow remover on the basis of the indication by the indicator lamp. As a result, the actual rotational speed of the engine can be matched to the target rotational speed.
In the auger-type snow remover described in Japanese Patent Laid-Open Publication No. 2002-142307, a machine body provided with a snow-removing work unit is moved along by a travel unit, and the snow-removing work unit is driven by an engine.
The auger-type snow remover (self-propelled work machine) described in Japanese Utility Model Laid-Open Publication No. 3-32617 will be described herein with reference to FIGS. 18A and 18B as an example of the abovementioned prior art. FIGS. 18A and 18B are schematic views of the conventional self-propelled work machine. FIG. 18A is a side view of the self-propelled work machine. FIG. 18B is a schematic view of the self-propelled work machine.
The conventional self-propelled work machine 100 (auger-type snow remover 100) is composed of a snow-removing work unit 101, a machine body 102 to which the snow-removing work unit 101 is provided, and a travel unit 103. The snow-removing work unit 101 is composed of an auger 111, a blower 112, and a shooter 113. The travel unit 103 is composed of a crawler. An engine constitutes a drive source 104 for driving the snow-removing work unit 101 and the travel unit 103.
In this auger-type snow remover 100, the type and accumulated amount of snow removed can be estimated by a control unit 123 on the basis of the rotational speed of the engine 104 detected by a speed sensor 121, and on the load torque of the snow-removing work unit 101 detected by a torque sensor 122. The control unit 123 controls the speed of the travel unit 103, auger 111, and blower 112 on the basis of the results of this estimation.
Specifically, the control unit 123 reduces the speed of the travel unit 103 and increases the speed of the auger 111 and blower 112 when it is estimated that the snow type is icy and the snow coverage is low in the portion of snow removed. The control unit 123 also reduces the speed of the travel unit 103, auger 111, and blower 112 when it is estimated that the snow type is regular (soft snow or the like) and the snow coverage is high.
However, it is often the case that the snow type or coverage of the portion of snow removed varies continuously. As in the auger-type snow remover 100, when the load exerted on the snow-removing work unit 101 varies according to the snow type or amount of the portion of snow removed, merely changing the travel speed of the travel unit 103 will cause frequent repetitions of deceleration and acceleration with each variation of the load. For example, frequent significant changes in the travel speed during snow removal are bothersome to the operator. Improvements can be made in order to increase ease of operation. Regardless of the travel speed or the size of the load, the speed is sometimes too low when a simple deceleration to a constant travel speed is made each time the load increases, and there is potential for improving the ease of operation in this aspect as well.
There is also potential for making it easier to operate an auger-type snow removers such as the one described in Japanese Laid-open Patent Application No. 2004-278055, wherein operation is made inconvenient by the fact that the operator must frequently change the travel speed each time the load on the auger changes according to the snow type or amount of the portion of snow removed.
Since snow is removed at low temperatures, a relatively long time is required from the time the engine 104 is started until the warm-up operation (warm-up) is completed. The operator is therefore inconvenienced by the need to wait without removing the snow until the warm-up is completed. The warm-up operation performed manually by the operator involves first closing a choke valve, and then gradually opening the choke valve according to the warm-up state.
Because of the inconvenience of this operation, the use of an automatic choke is considered. An automatic choke is a device for automatically opening and closing the choke valve according to the temperature state of the engine. The device is also referred to as an auto-choke. In other words, a configuration may be adopted in which the valve travel of the choke valve and throttle valve of the engine are adjusted by an electronic governor. Various types of such devices are known.
For example, a work machine equipped with an engine in which the valve travel of the throttle valve is adjusted by an electronic governor is described in Japanese Patent No. 2832610. In the work machine described in Japanese Patent No. 2832610, an electronic-governor throttle valve is provided to an engine mounted to a rice planting machine or a cultivator. An automatic choke may be fixed on the engine of this work machine.
An additional description will now be given with reference to FIGS. 18A and 18B. A case can be envisioned in which an electronic governor is employed for adjusting the valve travel of a choke valve and throttle valve in the engine 104 of a conventional auger-type snow remover 100. The warm-up operation is performed with the choke closed, but the load placed on the engine 104 is large when the auger-type snow remover 100 is moved forward and snow removal is started in this state.
This situation is particularly likely to occur when the travel unit 103 is made separate from the drive system actuated by the engine 104 and is part of a drive system actuated by an electric motor. This occurs because the auger-type snow remover 100 is caused to advance despite of the fact that the engine 104 is still warming up.
The performance of a common snow remover in a case in which the travel unit 103 is part of an electric motor drive system will be described with reference to FIG. 19.
FIG. 19 is a diagram describing the performance of the conventional auger-type snow remover. The drawing shows the performance of the auger-type snow remover. The elapsed time is plotted on the horizontal axis, the throttle valve travel Str is plotted on the vertical axis on the left side of the diagram, and the choke valve travel Cr and the actual speed Tr of the electric motor are plotted on the vertical axis on the right side of the diagram.
The choke valve travel Cr is 0% at t1 and 100% at t2, where t1 is the time at which the engine is started, and t2 is the time at which the warm-up operation is completed. In other words, the choke valve travel gradually increases in size from 0% to 100% according to the warm-up state.
The throttle valve travel Str increases sharply when the snow-removing work unit is driven by the engine while the travel unit is moved forward by the electric motor at about the same time as the engine is started. This is because a large load is placed on the engine. In other words, the throttle valve travel Str is unnecessarily large.
When a configuration is adopted in which the actual speed Tr of the electric motor is reduced relative to the increase in the throttle valve travel Str, the actual speed Tr of the electric motor sharply decreases with rapid increase in the throttle valve travel Str. Snow is therefore not removed by the auger-type snow remover. There is thus no point in starting to remove the snow early during warming-up of the engine. This technique leaves room for improving the ability to remove snow.
The auger-type snow remover (self-propelled work machine) described in Japanese Laid-open Patent Application No. 2002-142307 will next be described with reference to FIG. 20. FIG. 20 is a schematic diagram of a conventional self-propelled work machine.
The conventional self-propelled work machine 200 (auger-type snow remover 200) is described as being provided with a snow-removing work unit 203 composed of an auger 201 and a blower 202, an engine 205 for driving the snow-removing work unit 203 via a clutch 204, left and right travel units 206 and 206 composed of crawlers, left and right electric motors 207 and 207 for driving the travel units 206 and 206, left and right brakes 208 and 208 for applying braking to the travel units 206 and 206, a control unit 209 for controlling the electric motors 207 and 207 or the brakes 208 and 208, and various types of operating members 211, 213, 214, and 214 for issuing operating signals to the control unit 209.
The travel of the throttle valve 212 of the engine 205 can be adjusted by operating a throttle lever 211. The rotational speed of the engine 205 increases as the throttle valve 212 is opened.
The control unit 209 controls the direction or speed of rotation of the left and right electric motors 207 and 207 according to the operation of an accelerator lever 213, and controls the left and right brakes 208 and 208 according to the operation of speed adjustment levers 214 and 214.
The auger-type snow remover 200 thus configured is a type of work machine in which a snow-removing work unit 203 is driven by an engine 205, and travel units 206 and 206 are driven by electric motors 207 and 207.
By operating a clutch operating member (not shown) to switch on the clutch 204, the snow-removing work unit 203 can be driven by the power of the engine 205 to remove snow. The rotational speed of the engine 205 is reduced according to the size of the load placed on the snow-removing work unit 203. The travel of the throttle valve 212 is automatically increased according to the degree of speed reduction in order to maintain the desired rotational speed. The control unit 209 causes the travel speed of the travel units 206 and 206 to decrease by reducing the speed of the electric motors 207 and 207 according to the reduction in the rotational speed of the engine 205 or the increase of in the travel of the throttle valve 212. Specifically, the auger-type snow remover 200 is propelled at the travel speed that corresponds to the snow removal load.
The characteristics of a common auger-type snow remover 200 in a case in which the travel units 206 and 206 are driven by electric motors will next be described with reference to FIGS. 21A and 21B with reference to FIG. 20.
FIG. 21A is a timing chart in which the elapsed time is plotted on the horizontal axis, and the travel Str of the throttle valve 212 is plotted on the vertical axis. FIG. 21B is a timing chart in which the elapsed time is plotted on the horizontal axis, and the actual speed Tr of the electric motors 207 and 207 is plotted on the vertical axis. The characteristics shown in FIGS. 21A and 21B are correlated between these two diagrams.
At time t21 when the clutch 204 is switched on, a large load is placed on the snow-removing work unit 203 for an extremely short time. As a result, the rotational speed of the engine 205 begins to increase after sharply decreasing for a short time. The travel Str of the throttle valve 212 also changes rapidly for a brief time in conjunction with the rapid change in the rotational speed of the engine 205. The control unit 209 causes the actual speed Tr of the electric motors 207 and 207 to change rapidly for a brief time according to the sudden change in the rotational speed of the engine 205 or the sudden change in the travel Str of the throttle valve 212.
Since the travel speed of the auger-type snow remover 200 is generally low, an operator who is relatively skillful at removing the snow is not inconvenienced at all by this degree of variation in the travel speed. On the other hand, to a novice operator unskilled at removing the snow, travel should preferably be made as stable as possible in order to increase workability.
The same applies at time t22 or t23 when the travel Str of the throttle valve 212 is adjusted by operation of the throttle lever 211. This is because the throttle lever 211 is not necessarily operated smoothly by a novice user unaccustomed to its operation.
Therefore, a technique is needed that is capable of further enhancing the ease of operation of a self-propelled work machine provided with an engine-driven work unit in a machine body that can be self-propelled using an electric motor.